1. Field of the Invention
This invention relates to a novel sintered article of diamond for use in tools and the like and a method for the production thereof.
2. Prior Art Statement
Diamond possesses many outstanding qualities originating in its covalent bond. Owing to the enormous capacity for covalent bond, it possesses an extremely small self-diffusion coefficient and exhibits little sintering property. Diamond entails the disadvantage that it is stable only under high pressure (metastable under the conditions of normal room temperature and normal pressure) and undergoes graphitization at elevated temperatures when the pressure is insufficient. For diamond to be sintered in the absence of a sintering aid, it requires a very high temperature of 2,440.degree. K. (about 2,170.degree. C.) and, at the same time, necessitates an extremely high pressure of 8.5 GPa (85,000 atmospheres). These conditions are extreme and are unfit for commercial production of any material.
As a means for producing a sintered article of diamond without entailing graphitization, there is known the method which comprises mixing diamond powder with about 20% by volume, based on the total volume of the diamond powder and aids, of a metal such as cobalt, iron, or nickel which is capable of acting as a solvent for diamond and then sintering the resultant mixture. In this method, the sintering proceeds in a liquid phase with the aforementioned metal functioning as a binder for diamond. This sintering can be attained under conditions of relatively high feasibility, namely under a low pressure on the order of 5 to 6 GPa and at a low temperature not exceeding 1,600.degree. C. [Japanese Patent Publication SHO 39(1964)-20483 and H. Katzman and W. F. Libby, "Science", 172, 1132 (1971)]. In the sintered article obtained by this method, diamond powder is bound to one another mainly with a low melting point metal phase. When this sintered article is exposed to a high temperature, therefore, it suffers from a conspicuous deterioration of mechanical property due to softening of the metal phase. During the course of the sintering, diamond is dissolved and recrystallizes and this phenomenon induces abnormal growth of powder of diamond and formation of pools of molten metal. The diamond powder and molten metal pools cause a decline in the strength of the sintered article. When the sintering is carried out in the presence of the aforementioned solvent metal as an aid, the degradation of the high-temperature strength of the sintered article cannot be precluded.